1. Field of the Invention
This invention relates to electrically heated pads, and more particularly to electrical pads having a heating element that eliminates magnetic field being emitted from heating wires.
2. Discussion of the Related Art
It has been discovered that electromagnetic fields affect biological matter and, consequently, cause health problems in human. Such health problems may be linked to cancer and other ailments. The electromagnetic fields are very common in modern society where consumer electric appliances are frequently used at homes and offices. Commonly, electromagnetic fields are generated by power lines and transformers. The electric blankets or pads which we use to keep warm also generate one or more forms of electromagnetic field, which includes magnetic field. In fact, some studies have shown that there may be a direct correlation between electric blankets and certain medical conditions.
As it is well known that the electromagnetic field permeates through practically every kind of substances with the exception of ferromagnetic materials. Thus, attempting to block electromagnetic field in such appliances as electric blanket using ferromagnetic materials may not be practical due to its cost.
For example, FIG. 1 is a block diagram of a temperature controller and a heating element of a conventional electric blanket. The temperature controller 10 includes a bimetal thermostat switch 12 which can be set to appropriate temperatures to provide current to the heating element 20. Alternatively, as shown in FIG. 2, the temperature controller 10 may include an integrated circuit switch 14 instead of the thermostat switch 12 to provide the same function. The common characteristic of both FIGS. 1 and 2 is that the output voltage Vout at node 18 comprises a sinusoidal wave alternating current (AC). When using a conventional household line voltage, the Vout will be 60 Hz wave with peak voltage slightly higher than 120V. The output voltage Vout is connected to the heating element 20. The heating element 20 is made with a resistive wiring 22 which produces heat when electricity is applied. Furthermore, the devices shown in FIGS. 1 and 2 use a resistive wiring 22 which is arranged to produce magnetic and electromagnetic field which may be harmful to human.